Systems for estimating the location of vehicles in space are known. Some of these systems rely upon the Global Positioning System (GPS) to obtain an accurate position estimate of a space vehicle of interest. The GPS system comprises a constellation of satellites that each revolve about the earth in a fixed orbit. The GPS system presently comprises 24 satellites, including 21 navigational space vehicles (SVs) and 3 active spare vehicles, that revolve around the earth in 12 hour orbits. The SVs are arranged in six orbital planes that are equally spaced from one another (i.e., neighboring orbital planes are 60 degrees apart).
The GPS system also includes a network of ground based tracking stations that are located at various locations around the world. These tracking stations receive signals from the navigational SVs and transfer the signals to a central location. The central location uses orbital models to compute orbital data (known as ephemerides) and clock corrections related to the navigational SVs. This information is then uploaded to the SVs via a radio frequency uplink. The navigational SVs then broadcast portions of the ephemerides for use in determining the position of a point of interest (POI). The transmitted information is known as the navigation message.
To estimate the position of a point of interest, the navigation message from four or more SVs must be received at the POI and appropriately processed. These functions are generally performed by a GPS receiver located at the POI. The GPS can estimate the position of virtually any point that is capable of receiving the navigation message from at least four navigational SVs. The POI can be located on the earth's surface (such as on land or on a vessel at sea) or above the earth's surface (such as aboard an aircraft or space vehicle).
In many applications, the accuracy bf the location estimate achieved by the above-described system is inadequate. For example, the ephemerides transmitted by the navigational SVs normally provide an accuracy from 15-100 meters. This would not be acceptable in high-accuracy applications, such as some remote sensing applications, that require an accuracy of 10 cm or less. Therefore, methods must be developed to increase the accuracy of the estimates to service these applications.
One known method uses a post processing mode of operation to achieve highly accurate GPS estimates for locating satellites in space. This method has been successfully implemented to provide highly accurate satellite location estimates in NASA's Topex oceanographic mission. The method comprises collecting and archiving GPS tracking data from the worldwide network of tracking sites. At the same time, the user satellite to be located collects and stores GPS tracking data from an on-board GPS receiver. A Mission Control Center obtains the GPS tracking data from the tracking sites and the user satellite and processes the data to obtain orbit data for both the GPS satellites and the user satellite. Because information from both the tracking sites and the user satellite is processed concurrently (i.e., a joint solution is obtained), higher position accuracy is achieved. A description of a system which uses this approach is found in "Precise Tracking of Remote Sensing Satellites With the Global Positioning System" by Yunck et al., IEEE Transactions on Geoscience and Remote sensing, vol. 28, no. 1, January 1990, which is incorporated by reference herein. Because the data from the tracking sites is only available at specific times (e.g., in 24 hour blocks), the precise orbit information resulting from the processing is typically available a day or two after the observation. While this delay is acceptable in some applications, other applications require data that is more current.
In another known method, differential techniques are used to achieve accurate orbit estimates for a space system. In a differential system, one or more reference stations on the ground receive navigation messages from multiple GPS satellites within view and calculate corrections based on the known location of the reference station(s). The reference station(s) then broadcasts the corrections to a surrounding area for use in correcting the position solutions calculated by roving GPS receivers in the area. Because the correction is based on the location of the reference station, the accuracy of this method decreases as the distance between the roving GPS receiver and the reference station increases. In addition, the method requires a means for broadcasting the corrections over a relatively large area, which can be costly.
Therefore, a need exists for a method and apparatus for estimating the position of a vehicle in space that is both highly accurate and capable of providing results that are more current than systems of the past. In addition, a need exists for a system that provides an accuracy that does not depend on the distance between a GPS receiver and a reference station. Further, a need exists for a positioning system capable of high positional accuracy without the need for costly correction broadcast means. It is desirable that the system be capable of providing results in substantially real time.